Aphids or "plant lice" are soft bodied pear-shaped insects generally less than 1/8 inch long. Their color varies according to species, however the common ones are usually green, yellow or black. They usually occur in colonies on the undersides of tender terminal growth. Heavily-infested leaves can wilt or turn yellow because of excessive sap removal. While the plant may look bad, aphid feeding generally will not seriously harm healthy, established trees and shrubs.